Glass cannons, in my experience. They can really dish out the damage fast, especially if there's a few of them and they get flanks, or if you're fighting a bunch of PCs who all have darkvision and so didn't bother bringing along a way of making light. Note that even though they can Hide as a bonus action, they don't have Hide In Plain Sight, so they'll still need something to hide behind (or some darkness to hide in, unless they're facing PCs with darkvision and depending how you as a DM handle hiding)
Against prepared PCs, or PCs who happen to be of the right subclasses, they can go down very fast and hard. Paladins love them, low AC means you can swing for the fences with Great Weapon Master, and your smites do extra damage because they're fiends, and on top of that it's radiant damage that they are vulnerable to. And paladins have all sorts of really nice spells (Protection from Evil/Good, Divine Favor just as two examples, but stuff like Daylight makes an appearance at higher level) that are tailor made for picking on these guys. Light domain clerics love them too, Radiance of the Dawn will make a mess of these guys.
A bit hard to use, I think. In small numbers against low level parties, they could be really lethal. As mobs against more powerful parties, they can get stomped.